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Abstract

Different immunoglobulin (Ig) heavy chain (H) isotypes have distinct functions, but so far it is unclear if Ig light (L) chains follow the same pattern. It is usually assumed that form follows function; but if this is true, then why have different IgL isotypes with no known functional differences? In this study we investigate IgH and IgL isotype preferential binding and complementary determining region (CDR) lengths to try to address this question using the African clawed frog, Xenopus laevis, as a model. Amphibians exhibit IgH isotype class switch at a single IgH locus and have an additional, more divergent, IgL isotype (σ) plus the two found in mammals (λ and κ). We used quantitative PCR (qPCR) analysis of IgH isotype of B cells sorted by surface IgL isotype expression to find evidence of preferential use of IgL isotype by IgH isotype. We found a relative skewing in the Igκ cells for IgY, in the Igλ cells for IgX, the Igσ cells for IgM, and corroborated published immunoprecipitations showing that IgY and Igσ do not pair with gene expression data of the IgL isotype sorted cells. Our data also suggests that the exaggerated CDR1 of IgHV families III and VII and the long CDR2 of Igσ may cramp IgH CDR3, making the IgHV III/VII-Igσ pairing less common. While these data do not resolve the conundrum of multiple IgL isotype maintenance in vertebrates, they do show that in a tetrapod with several IgH and several IgL isotype options, IgL isotype use is not random.